1. Field of the Invention
The present invention relates to a wet image forming apparatus and, more particularly, to an image fixing unit for a wet image forming apparatus, designed to improve the quality of a final image fixed on paper by smoothly evaporating vapor of a liquid carrier generated in the process of fixing.
2. Description of the Related Art
Wet image forming devices are generally constructed to develop desired images with a developer solution, such as a mixture of a liquid carrier and a toner comprised of electrically charged particle, and then to transfer the developed images to paper.
FIG. 1 illustrates a fixing section of a conventional image forming apparatus. As shown, an image C1 transferred on paper C through a transfer process is fixed on the paper C when the paper C passes through a fixing nip or gap between first and second fixing rollers 1 and 3.
Because the first and second fixing rollers 1 and 3 have heating element mounted therein and are pressed relative to each other at proper temperature and pressure, a liquid carrier contained in a toner passing through the fixing nip under the proper temperature and pressure is evaporated. Simultaneously, an adhesive force between the paper and the toner is generated.
Before passing through the fixing nip the toner contains a liquid carrier. The amount of liquid carrier in the toner ranges from at or about 10% to at or about 80%. While the toner passes through the fixing nip under the proper temperature and pressure, the liquid carrier is evaporated at the fixing nip, but typically the vapor fails to escape smoothly. This causes marks, such as a ripple pattern, to be left on the image fixed on the final paper C, thus resulting in poor image quality.
In order to solve this problem, one prior art solution has been disclosed in U.S. Pat. No. 5,465,146, the entire contents of which are disclosed herein by reference.
As shown in FIG. 2, the configuration disclosed in U.S. Pat. No. 5,465,146 is composed of a drying section 11 for drying the paper C onto which the images C1 are transferred in a non-contact manner, and a fixing section 13 for heating and pressing the paper C to fix the images C1. The drying section 11 includes a first roller R1 in which a first heater H1 is mounted, and a drying belt B traveling between the first roller R1 and a second roller R2. The fixing section 13 includes the second roller R2, and a third roller R3 having a second heater H2 mounted therein and rotating in close contact with the second roller R2 with the drying belt B in between second roller R2 and third roller R3.
This structure, however, has a drawback in that the drying process has a low thermal efficiency because the drying section 11 is designed in a non-contact heating manner. Specifically, the paper C is heated in the non-contact manner through a radiant heat of the drying belt B heated by the first heater H1, so that although the surface temperature of the drying belt B is high, the paper C supplied with the heat of the drying belt B has a temperature falling far short of the surface temperature of the drying belt B.
To solve such a drawback, another prior art solution has been disclosed in Korean Patent Publication No. 2003-015541 (published on Feb. 25, 2003), the entire contents of which are incorporated herein by reference.
As shown in FIG. 3, the configuration disclosed in Korean Patent Publication No. 2003-15541 is composed of a charging section 10 which is installed on a feed path of paper to apply an electric potential that brings toner into close contact with the paper, a drying section 20 which heats the paper C in direct contact with the drying section 20 to dry the images, and a fixing section 30 which heats and presses the paper to fix the images.
The drying section 20 includes a drying belt 21 that moves along an endless path between a drying roller 22 and a supporting roller 23, a backup roller 24 that rotates in close contact with the drying belt 21, a heater 22a mounted in the drying roller 22, and a regeneration roller 25 having another heater 25a mounted therein for evaporating the solvent absorbed into the drying belt 21 by contact with the drying belt 21.
Because the surface of the paper onto which the images are formed (hereinafter called a “image surface”) is configured to directly contact the drying belt 21, however, the evaporated liquid carrier remains between the drying belt 21 and the paper C and cannot escape. This is a problem in that this also generates the aforementioned image defect of a ripple pattern on the final images of the paper which has passed through the fixing section 30.